While most lube base oil is made from crude oil (and is sometimes referred to as virgin base oil), it is well known that used lubricating oil is an excellent feedstock for re-refining into base oil. Re-refining technologies include most commonly either distillation, thermal de-asphalting, or solvent de-asphalting, which create one or more intermediate liquids, certain of which are then further upgraded to create marketable base oil, most commonly either by clay treatment, hydrotreatment, or solvent extraction. In the 1960s, acid/clay treating was prevalent but was discontinued due to extensive by-product solids creation that became ground pollutants, creating many super-fund sites that incurred massive clean-up costs.
Technologies such as distillation, thermal de-asphalting, or solvent (often propane) de-asphalting may be referred to as recycling technologies since the major intermediate products created by these processes can be sold as a cleaner burning fuel oil (such as a marine diesel oil or MDO) or sold as vacuum gas oil (VGO), which can be feedstock to a conventional crude oil refinery. The intermediate products (MDO or VGO, which may also be referred to as “intermediate lube distillate”) created by recycling technologies are unfinished and typically unsuited for use as lubricants without further improvement. Technologies such as clay treatment, hydrotreatment, or solvent extraction may be termed finishing technologies since these are used to “finish” the quality of the intermediate lube distillate into marketable base oil. When a recycling technology and a finishing technology are coupled together, they are generally referred to as a re-refining technology.
Solvent de-asphalting has been applied in various modes including as a standalone step in separation of asphaltenes and an intermediate lighter less contaminated material which can then be sold as a burner fuel or further processed into a base oil. U.S. Pat. No. 5,556,548 and U.S. Pat. No. 6,174,431 (Interline Hydrocarbons) disclose a process wherein used oil is mixed with a solvent (such as propane, acetone, isopropyl alcohol, or a suitable hydrocarbon) which is lighter than the used oil and the solvent carries above with it a substantially non-asphaltic stream, leaving the heavier asphaltic materials then separated out as a residual at the bottom. The oil and solvent are passed through an activated carbon filter to remove metallic compounds and the solvent is then evaporated and re-captured for re-use. Solvent de-asphalting logically is applied as an alternative process to distillation and as with distillation, the material that is created is not suitable for sale as a base oil. However, unlike distillation, solvent de-asphalting will leave relatively high level of metals remaining in the created product. For this reason, solvent de-asphalting is far less common a recycling technology than is distillation. In the instant invention, the non-asphaltic material created by solvent de-asphalting a used oil is referred to as intermediate lube distillate, even though technically the material is created by a solvent based process versus a distillation process.
There are other recycling and finishing technologies beyond those listed above. The key point is that an intermediate material is generally first created and this intermediate material is then further upgraded. For example, the intermediate lube distillate created for finishing is generally bounded by a boiling range on the low end in the 500 to 550 degree Fahrenheit (° F.) and below range (which removes lighter liquids and diesel) and, on the high end in the 1050 to 1100° F. and above range where heavier asphaltic type materials are removed. (Some recycling processes may use boiling ranges extending well into the mid to upper 600° F. range which would then include a spindle oil cut along with the lighter liquids and diesel that is removed prior to the finishing stage.) Knowledge of these factors and areas, along with fouling and corrosion issues and other variations on used oil distillation, are well known to one of ordinary skill in the art and do not require further explanation. Accordingly, the term describing an oil to be further upgraded by finishing into a base oil is “intermediate lube distillate”. Intermediate lube distillate is preferably defined to include an oil derived in large part from used lubricating oil utilizing any process which has removed a majority of the lighter fractions that generally occur at 500 to 550° F. or below and removed a majority of the heavier fractions that generally occur at 1050 to 1100° F. and above. An intermediate lube distillate can also be derived from crude oil. For example, in refining most commonly in the past an intermediate lube distillate has been created by processing crude oil through atmospheric distillation, vacuum distillation, solvent extraction, and solvent de-waxing units. The intermediate lube distillate that is created by these units is then hydrotreated to make a virgin base oil. In addition, sometimes solvent de-asphalting of the vacuum distillation residuum (the heaviest boiling point components emerging from the bottom of the vacuum distillation column) creates a non-asphaltic stream that is further processed into an intermediate lube distillate that is then hydrotreated to make a virgin base oil.
In the past, re-refined base oil has been viewed as inferior in quality to virgin base oil, although as re-refining technologies have improved, this is no longer accurate. Used lube oil quality has also increased markedly with the dramatic improvement in the finished lubricants' quality from which the used oils are generated. Up until 2010, about 95% of all U.S. base oil was produced in large refineries (which use crude oil as feedstock) with specially designed process units creating virgin base oil from select streams within the refinery. Only about 5% of all base oils then produced in the United States were re-refined base oils. As re-refining economics, technology, and used oil product quality improved, more re-refineries have been announced or been constructed and, as of 2013, approximately 10% of all U.S. base oil is forecasted to be supplied by re-refineries.
Three key determinants of base oil quality are lower sulfur (to reduce harmful emissions), increased saturates (which improve oxidation stability, reduce sludge and deposit formation, and have other beneficial characteristics) and a higher viscosity index (VI). VI measures the rate of change in viscosity in response to a change in temperature. With lube oils, the lesser the change the better, and a higher VI number indicates less viscosity change in response to temperature change, and thus higher quality. Saturates are well known to have a smaller change in viscosity in response to temperature than aromatics and polar compounds, and thus have higher VIs. It is thus well known that a higher level of saturates in base oil increases quality as the more saturated base oils achieve higher VIs. In addition, within the saturates, paraffinic components are known to have a higher VI than naphthenic components.
Table 1 below provides the American Petroleum Institute's (API1509) classification of base oils by groups of generally increasing quality, based on sulfur, saturates, viscosity index (VI), or means of manufacture. Groups I, II, and III are mineral oils (i.e., thus made from crude oils), whereas Group IV and above are made from petro-chemicals (thus derived ultimately from natural gas). Most notable in Table 1 below is the requirement that Group III base oils equal or exceed a 120 VI. Base oil quality is increased as the group levels increase, and this is reflected in higher market pricing of these base oils.
TABLE 1API Base Oil Group ClassificationsViscosityGroupIndexSaturates and SulfurOtherI80-120<90% and/or >=0.03%II80-120>=90% and <0.03%III≧120>=90% and <0.03%IVPAO Poly Alpha OlefinVPoly Esters (others)VIEurope Only (ATIEL)PIO (Poly Internal Olefins)
Increasingly strict United States government regulatory requirements requiring reduced emissions and improved fuel economy are translated into specific performance requirements for automobile motor oils by a tripartite industry committee called ILSAC, which consists of automobile manufacturers, additive manufacturers, and base oil producers. The latest performance standards established by ILSAC are GF-5 and these became effective on Oct. 1, 2011. The ILSAC standards specify tests which focus on ensuring the finished lubricant (the base oil and additives together in combination) will achieve certain explicit measures of fuel efficiency, catalyst compatibility, minimum levels of wear and deposits, and most particularly of importance in the lighter lubricant viscosity range, a limitation on volatility. Over the years, the increased GF standards have translated into enormous pressure on both virgin base oil producers and additive manufacturers to improve their products by creating higher quality base oils and higher performance additive packages to create higher quality motor and engine oils. For base oils today, this means increased demand for higher quality Group II base oils and, in some cases, even Group III base oils. This ever upward quality trend has continued for many years and virgin base oil producers have been forced to upgrade their facilities to produce higher quality base oils to meet the more stringent emissions and fuel economy standards. Most of the smaller, less efficient refineries producing virgin base oil could not justify the capital upgrades and either shut down their base oil plants within the refinery, or shut down the refinery.
Increased demand for Group III base oils coupled with a limited supply of Group III base oils has resulted in a sustained increased price premium for Group III pricing over Group II base oils. For example, in the U.S. Gulf Coast, the average historical price for the three year period ending Jul. 31, 2013, for Group III base oil is $5.30 per gallon, Group II base oil is $4.24 per gallon, and Group I is $3.86 per gallon. Thus, Group III traded for $1.06 per gallon (or an average 25% price premium) over Group II and $1.44 per gallon over Group I (or an average 37% price premium). Despite the substantial price premium for Group III, it is generally cost prohibitive to upgrade existing virgin base oil plants and, in fact, there is currently no refinery (or re-refinery) in the United States producing Group III base oil. In Canada, one small plant produces Group III base oil which comprises the entirety of all current Group III production in North America (and is less than 2% of worldwide Group III production). Group III base oil production is concentrated in the Far East and Middle East, which have 61% and 26% of the worldwide Group III capacity, respectively, thus together comprising 87% of the world's Group III capacity. Newer Group III plants outperform most older U.S. plants which were not designed to produce Group III based oils.
While selection of a recycling technology will somewhat affect base oil quality, for a given used oil feedstock, selection of the finishing technology (clay treatment, hydrotreatment, or solvent extraction) is the largest single factor affecting re-refined base oil quality. A comparison of the three most common finishing technologies used in re-refining is shown in the table below.
TABLE 2Comparison of Most Common Re-refining Finishing TechnologiesFinishingBase OilCapitalTechnologyBase Oil QualityYieldCostOperating CostClayLow - higher sulfurGood -Lower:Low - clay useTreatmentLow - low saturates(95%+)Index = 1and moderateLow - VI smalltemperatureimprovementHydro-Highest - lowestBest -Highest:Highest - duetreatmentsulfur(98%+)Index =8 to hydrogen, Highest - highesthigh pressure saturateand highHighest - VI muchtemperatureimprovedSolventLow - higher sulfurWorst -Moderate: Low - someExtractionMid - moderateBase oilIndex = 3heat,saturateslost toelectricity,Low - VI improved,extractchemicals, but limited by (80%-plus smallyield loss90%)solvent losses
Hydroprocessing is considered to be the gold standard and is used to make Group II base oil but requires a capital cost far in excess of that required for solvent extraction or clay treatment. Under more severe process conditions such as temperatures above 700° F. and pressures above 1,500 psig, hydroprocessing is referred to as hydrocracking, and is used to make Group III base oil. Hydro-cracked base oils, even while exhibiting excellent volatility, anti-oxidation, viscosity response to temperature (viscosity index or VI), and low temperature characteristics, typically have poor solubility and lubricity qualities which then impair the hydro-cracked base oil's ability to be mixed with additives. Explanations of these impairments are well presented in U.S. Pat. No. 7,655,605 (Rosenbaum/Chevron) and in the July 2009 STLE magazine article “New Base Oils Pose a Challenge for Solubility and Lubricity” (pages 34 through 39). In addition hydrocracking results in material yield reductions with up to 40% of the material being lost as a lubricant. While hydro-cracking is employed in many large refineries to make virgin base oils, there are no known re-refineries employing hydro-cracking because, in the current market, the capital cost is too high and the yields of lube oil are too low to economically justify the large investment.
Hydrotreatment is commonly applied in re-refining under moderate hydrotreatment processing severities (less than 700° F. and below about 1,500 psig, and space velocities in the range of 0.5 to 2.0) since it is now capable of routinely producing re-refined base oil with VIs in excess of 110, saturates well in excess of 90%, and sulfur and nitrogen below 200 PPM, thus creating a higher quality Group II base oil. However, hydrotreatment is not yet demonstrated to be capable of cost effectively making base oils over a range of viscosities each of which has a 120 VI or more, and most particularly in the lighter viscosities of 100 to 150 SUS, although this may change in later years with continued quality improvement of used oil feedstocks.
To understand the limits of hydrotreatment, it is important to understand the means by which hydrotreatment improves base oil, which in turn requires some knowledge of base oil composition. Base oil may be classified into three groups of compounds: saturates, aromatics, and polars. Thus whether by volume or by mass, Base Oil=Saturates+Aromatics+Polars (olefins are excluded as they are not likely to be material). Aromatics and polar compounds are low VI liquids whereas saturated compounds are higher in a range of VIs. Thus, higher levels of aromatics and polars in base oil will reduce the base oil's VI, whereas increased saturates in base oil will increase VI. Under elevated temperatures and pressures, over a certain residence time, in the presence of hydrogen and over a catalyst, hydrotreatment converts much of the aromatics and polar compounds to saturated compounds, which may be paraffinic or naphthenic, thereby resulting in an increase in VI. Although both paraffins and napthenes are saturates, naphthenes have lower VIs than paraffins. Thus, the higher the proportion of paraffinic components to naphthenic components, the higher will be the resultant VI, which is desirable since higher VI lube oils have higher value.
Conversion of aromatics and polar compounds to saturates in hydrotreatment is primarily bounded by temperature, residence time, and pressure (leaving aside catalyst selection, hydrogen purity, and flow rate) with implications for base oil yields and capital cost. For example, there is an upper bound on temperatures above about the 650° F. to 700° F. range where increased residence time results in increased cracking and thus yield loss and coking of hydro-treating catalysts. Below the cracking temperatures, residence time can be increased (or alternatively stated, space velocity lowered), but there is a practical limit on results that can be achieved by increased residence time. Reaction dynamics limit product improvement to temperatures and residence times above which net conversion of aromatics and polars to saturates will begin to reverse.
Since the effectiveness of higher hydrotreatment temperature and longer residence time are effectively bounded, pressure is a third variable to consider. Creating a VI of 120 or more (thus a Group III) in base oil will often require pressures in the 2,000 to 3,000 psig range, which moves well above hydrotreatment and into hydrocracking ranges. The core challenge of higher pressure hydrotreatment is that when pressure is increased, particularly above 1,500 psig, there is a step change up in capital costs caused by increased cost of materials and construction. This is triggered by moving from pressure class 900#, which is rated to about 1600 psig (assuming carbon steel and 700° F.), to pressure class 1500#, which is rated to about 2600 psig (again assuming carbon steel and 700° F.). In addition, while certainly producing a 120+VI base oil, the higher pressures and temperatures used in hydrocracking increase cracking and thus increase base oil yield loss and incur more catalyst coking.
Under the moderate severities of hydrotreatment, capital costs and product yields are preserved in an economic range. However, based on today's typical used oil feedstocks, conventional hydrotreatment is limited in its ability to reduce aromatics and polars, and create higher VI saturates, which generally limits the upper bound VI to below 120 (thus not achieving Group III). While in some cases with certain feedstocks a higher VI can be achieved for some base oil fractions (more particularly heavy lube oils which tend to have higher VIs to begin with), re-refiners using hydrotreatment have not yet been able to consistently produce Group III base oils across the viscosity range, and thus must depend heavily on obtaining high feedstock quality, which is uncertain and expensive to segregate in most used oil gathering operations. Thus, based on all currently known operating technologies, it is not economically feasible with current used oil feedstocks to consistently create Group III base oils over a range of viscosities solely by hydrotreatment of used oils.
An alternative finishing technology is solvent extraction, which is commonly used to extract aromatic and polar compounds from various feedstocks. Solvent extraction can be applied to intermediate lube distillate to make base oil, but there is a low base oil yield as the material which is extracted from the intermediate lube distillate is not marketable as base oil (since it is rich in aromatics and polar compounds, it is a low VI stream). Furthermore, and equally importantly, the base oil created using solvent extraction from the intermediate lube distillate cannot achieve a low enough sulfur level or sufficient color for many applications. Typically, solvent extraction has been used to make Group I base oil and it has been applied in crude refining and used oil re-refining applications for many decades. As noted in U.S. Pat. No. 8,182,672 (Exxon-Mobil Research), “The solvent extraction process selectively dissolves the aromatic components in an extract phase while leaving the more paraffinic components in a raffinate phase. Naphthenes are distributed between the extract and raffinate phases . . . . One can control the degree of separation between the extract and raffinate phases by controlling the solvent to oil ratio, extraction temperature, and method of contacting distillate to be extracted with solvent.” Increasing processing severity by increasing the solvent to oil ratio (e.g., the dosage) and/or temperature will increase base oil quality but will also increase the base oil yield loss to extract, which is a low valued material. This loss becomes cost prohibitive since base oil yield losses can exceed 20% to 30%, or even more, of the feed stream. Furthermore, the resultant base oil is generally still too high in sulfur for the transportation market which is a large, high value market application for lube oil. Due to its low yield and creation of inferior quality base oil, virtually all new plants targeting higher valued markets have rejected solvent extraction and instead adopted some form of advanced hydroprocessing. Clay treatment suffers from a number of issues including low base oil quality due to high sulfur and low saturate levels, and some yield loss and scalability challenges.
Some attempts have been made to achieve better performance in re-refining by coupling various technologies together. In U.S. Pat. No. 4,512,878 (Exxon-Mobil Research & Engineering Co.), a process is disclosed for using a heat soak (similar to thermal de-asphalting) followed by distillation and then finally hydrotreating the distilled material to create a base oil. In U.S. Pat. No. 4,125,458 (Exxon-Mobil Research & Engineering Co.), a process is disclosed for simultaneously de-asphalting and extracting a mineral oil feedstock. However, it is believed that such a process is not feasible for technical or economic reasons and was never commercialized. In U.S. Pat. No. 8,366,912 (ARI Technologies, LLC), an innovative process is disclosed by which solvent extraction is first employed to produce base oil and then the smaller aromatics/polars rich extract stream is further improved by hydrotreatment. Since the extract stream is a small portion of the total feed, this has the advantage of reducing required hydrotreatment capacity which is the most capital intensive finishing technology, although the primary cited advantage is improved base oil yield. Further disclosed, but not supported by test data, is that the base oil created directly by solvent extraction meets the Group I, II, or III specifications. In U.S. Pat. No. 7,261,808 (Shell Oil Company), a process is disclosed wherein solvent (propane) de-asphalting is first employed, followed by multiple stage hydrogen treatments, including demetallization, hydro-treating, de-waxing, and then further hydro-treating. While undoubtedly creating a highly improved base oil over what was originally contained in the used lubricating oil feedstock, this process is extremely capital intensive. Other patents that describe various means of distillation followed by solvent extraction include U.S. Pat. Nos. 4,021,333; 4,071,438; 4,360,420; 6,117,309; 6,319,394; 6,320,090; and 6,712,954.
Turning to the refining industry (which processes crude oil versus used oil), base oil processing technologies that were most commonly employed traditionally are distillation (atmospheric and then vacuum), followed by solvent extraction and then solvent de-waxing, and then finally, hydrotreatment. However, in place of the solvent de-waxing step, Chevron introduced hydro-isomerization where waxy compounds are changed into more beneficial compounds, which is then followed by hydro-cracking (replacing hydrotreatment). This process has been demonstrated to achieve highly positive results in multiple operating plants, producing extremely high quality base oils. An example of an alternative coupling of technologies is U.S. Pat. No. 6,325,918 (Exxon-Mobil Research & Engineering Co.) which first applies solvent extraction and then further improves base oil by hydrotreatment in what is called Raffinate Hydro-Conversion (RHC). Exxon-Mobil has successfully commercialized its RHC technology wherein solvent extracted base oils are then further improved by subsequent hydrotreating. In U.S. Pat. No. 3,781,196 a hydro-tracked liquid which has accordingly been processed at high processing severities (namely above 650° F. which is a temperature range which promotes severe cracking in lubricants and pressures above about 1500 psig) is then followed by solvent extraction with the objective of stabilizing the color and reducing the sludge formation of the resulting products. Other examples of base oil refining processes are found in U.S. Pat. Nos. 7,597,795; 7,655,605: 7,682,502; and 7,914,665, and there are also many others.
Well before the 1950s, which began introduction of hydrotreatment and the development of increasingly effective catalysts, at least one solvent based technique was advanced to separate naphthenic and paraffinic streams in order to create a paraffinic rich stream and thus a higher value lubricant. In U.S. Pat. No. 2,070,384 (dated Feb. 9, 1937) is described a process where solvent extraction using two solvents, one paraffinic and one naphthenic, are each used to create a paraffinic rich stream which has a high viscosity index. In U.S. Pat. No. 2,771,494 dated Nov. 20, 1956, a process is disclosed in which 2-pyrrolidone is used in solvent extraction for separation of naphthenic hydrocarbons from paraffinic hydrocarbons. This patent cites creation of a 95 percent pure paraffinic stream and a 99 percent pure extract stream (solvent free basis) although the feed is not a distilled mineral oil but rather an equal mixture of homogeneous paraffinic and naphthenic hydrocarbon liquids. In later years in the 1990s, U.S. Pat. Nos. 5,095,170, 5,120,900, and 5,107,056 disclose membrane separation processes whereby naphthenic hydrocarbons preferably migrate across a barrier to leave a more paraffinic concentrated stream remaining. However, none of the prior referenced technologies disclose any means or methods whereby crude oil processing technologies, recycling technologies, or finishing technologies are able to cost effectively achieve consistent production of high quality base oil, whether derived from crude oil, used oil, or a combination thereof. Blending even small portions of used oils into crude oil incurs high operating risks due to contaminants found in used lubricating oils that are not typically found in crude oils.
In view of the foregoing, there is a need for a system and method for efficiently generating a high VI base oil with a minimal loss of yield in the resulting base oil products.